The present invention relates generally to semiconductor wafer fabrication, and more specifically to a reactor system and wafer support for use during epitaxial growth of a semiconductor material on a semiconductor wafer.
In the semiconductor wafer manufacturing industry, thin epitaxial layers of semiconductor material, such as silicon or gallium arsenide, are grown on a surface of a semiconductor wafer. These epitaxial layers, commonly referred to as epilayers, form the material within which many modern integrated circuits are fabricated. In addition, many other devices, including optoelectric sensors, light emitting diodes, and micromachined mechanical devices, may be fabricated from epilayer material. As epilayers are a fundamental building block for many technologies, is critical that they be manufactured as efficiently and defect-free as possible, to reduce the cost and increase the quality of the epilayer.
Epilayers may be grown according to a variety of methods, including molecular beam epitaxy (MBE), vapor phase epitaxy (VPE), and liquid phase epitaxy (LPE). In a vapor phase epitaxial reactor, epilayer semiconductor constituents, such as silicon, gallium, arsenic, and germanium, and various dopants such as boron, phosphorous, arsenic, and antimony, are transported to the substrate surface as volatile species suspended in a vapor. Typically, the species are adsorbed onto the substrate at high temperature and diffuse across the surface to form the epilayer.
The VPE process takes place in a reactor including a heat energy source, such as radio frequency (RF) coils or heat lamps, and a susceptor. The susceptor typically is a solid graphite disk underlying and extending to the edge of the wafer and is substantially thicker than the wafer. One or more wafers are placed into the reactor directly on the susceptor, and the heat energy source is activated to heat the susceptor and the wafer. Where a RF heat energy source is used, the susceptor absorbs RF heat energy and conducts heat energy to the wafer. Where heat lamps are used, the susceptor absorbs heat energy and evenly distributes heat within the wafer, making the wafer less susceptible to temperature gradients within the reaction chamber.
After the wafer has been heated, gas containing the semiconductor constituents for epitaxial growth is introduced to the reactor through an inlet and flowed toward the wafer. Constituents are deposited on the front side of the wafer to form the epilayer. However, contact between the susceptor and the wafer inhibits gas flow to the back side of the wafer, such that constituents do not reach the back side and epilayer growth does not occur on the back side.
Several problems exist with reactors having susceptors. First, the thermal mass of the susceptor must be heated within the reactor along with the wafer before the epitaxial growth process may begin. For each wafer, it is common for the reaction chamber to be heated and cooled several times during the epitaxial growth cycle. For example, after a silicon wafer is inserted into the reaction chamber, the temperature is typically raised for a hydrogen bake of the wafer, which removes silicon dioxide contaminants from the wafer. The chamber is then cooled for epilayer deposition, and is again cooled before unloading of the wafer. After deposition, the chamber typically is heated again, and etch gases, such as hydrogen chloride, are flowed through the chamber to remove semiconductor material from the chamber and susceptor.
When producing epitaxial wafers on a mass scale, heating up and cooling down the susceptor consumes significant amounts of time and energy. In addition, the susceptors require frequent cleaning as semiconductor materials build up on the surface of the susceptors during the epitaxial growth process. Without cleaning, deposits may flake off and contaminate the epilayer growth process. In addition, susceptors must be replaced as their surfaces degrade from repeated epilayer deposition and cleaning, further increasing the materials costs associated with wafer manufacture.
Use of a susceptor for epilayer growth also may induce thermal stresses within the wafer. For example, where RF coils are used to heat the susceptor, the back side of the wafer adjacent the susceptor typically will be hotter than the front side of the wafer during epilayer growth, causing the wafer to bow. Thermally induced strain will develop in the lattice of the bowed wafer as the wafer cools.
Compared to other fabrication procedures, epilayer growth takes place under closely controlled conditions. A prior step in the wafer manufacture process may leave contaminants or imperfections on the surface of the wafer. One effect of the epilayer growth process is to remove these contaminants and correct these imperfections. However, reactors that grow an epilayer on only one side of a wafer, such as reactors that use susceptors, do not remove contaminants or perfect the imperfections on the back side of the wafer. These imperfections and contaminants on the back side may adversely affect a downstream circuit fabrication, test, or measurement procedure.
Where only the front side of a wafer is being coated with an epilayer, there is a risk that dopants within the substrate of the wafer will escape from the back side of the substrate at high temperatures during the epitaxial growth process, enter the gas flow, and contaminate the epilayer growth process on the front side of the wafer. This contamination process is referred to as autodoping, and is highly undesirable.
In addition, use of a susceptor in a reactor requires that the wafer be loaded onto the susceptor by a paddle that picks the wafer up by its top side. Some current reactors commonly utilize paddles that lift the wafer by creating a vacuum through direct suction or according to the Bernoulli effect. Loading and unloading through such vacuum operative paddles is slow, and consumes valuable cycle time per wafer.
A reactor system with an associated wafer support device is provided for use in the growth of an epitaxial layer of semiconductor material on a semiconductor wafer. The reactor system includes a reaction chamber including an inlet and an outlet configured to flow a source gas through the reaction chamber. The reaction system also includes a wafer support mounted at least partially within the reaction chamber, and a semiconductor wafer supported adjacent an outer edge by the wafer support. The wafer support device typically includes a hub and an arm extending outwardly from the hub. The wafer support device also typically includes a contact member coupled to the arm. In some embodiments a portion of the contact member extending downward relative to the back side of the wafer. The downwardly extending portion is configured to contact and support the wafer during epitaxial growth of semiconductor material onto the wafer. The contact member may be triangular or circular in cross section, and may be coupled to the arm via an upwardly extending support member and coupling member. In addition, the wafer support may include a hub and at least three arms extending radially outward from the hub. The wafer support may also include at least three contact members, each contact member being coupled to a respective arm. Each contact member includes a respective tip configured to directly contact the back side of the wafer adjacent an outer edge of the wafer and to support the wafer in a substantially horizontal orientation within the reactor system. The wafer support does not include a susceptor.